This invention generally pertains to an injection molding apparatus. More specifically, the present invention relates to a sprue bushing utilized in such an injection molding apparatus.
The invention is particularly applicable to a nozzle used in the injection of a relatively viscous fluid, such as a molten thermoplastic, and a relatively non-viscous fluid, such as a gas, into an injection mold during a process known as gas-augmented or gas-assisted injection molding. However, it will be appreciated to those skilled in the art that the invention has broader applications and may also be adapted for use in other injection molding environments where both a relatively viscous fluid, such as a plastic or wax, and a relatively non-viscous fluid, such as a gas, steam or liquid, are injected into a mold cavity.
Gas assisted injection molding processes are becoming widely known in the art. Such processes employ the steps of injecting a plasticized (melted) thermoplastic material under high pressure into a finite mold space but to a volume less than 100% of the mold space. Thereafter, an inert gas is injected under pressure into the plasticized material in order to fill the rest of the volume in the mold cavity. The gas enters the plasticized material and moves along the paths of least resistance therein. Such paths are normally in areas where the thermoplastic body is thicker and has slower cooling sections such as ribs, flow channels, chamfers, etc. In this way, with a suitably designed part, a continuous network of hollowed sections can be provided. The material displaced by the gas from the middle of the sections moves out to fill the remainder of the mold space. This network of gas channels provides a uniform pressure distribution system throughout the mold space during hardening and cool down of the molded part thus minimizing internal stresses. The outer surfaces of the thicker sections do not display sink marks because gas has cored them out from the inside and gas pressure holds the plastic material up against the mold surfaces during hardening. Sink in these sections takes place internally rather than on the exterior surfaces of the part. Since the pressure used for final filling of the part is confined to an area defined by the gas channels, the resultant force against the sections of the mold is relatively modest so that lower clamping forces on the mold are adequate.
Several types of such nozzles are known to the art. However, one disadvantage with such nozzles is the fact that they cannot be retrofitted onto a conventional plastic injection molding machine (i.e. of the non-gas assisted type) without extensive modifications. Also, most of the nozzles which are adapted to inject both a thermoplastic material and a gas into a mold cavity do not allow the discharge of the gas or other relatively non-viscous fluid back through the nozzle when such discharge is required. Instead, in these nozzle systems, the nozzle needs to be spaced away from the mold body in order to vent the gas out of the mold cavity. Those nozzles which are said to be adapted to vent the gas back through the nozzle are unsatisfactory because molten plastic, which remains in the nozzle or in the sprue of the mold body, is frequently vented back along with the gas. This can be deleterious to the gas lines in the nozzle and to the gas piping and valves downstream from the nozzle. Also, if such plastic solidifies in the gas lines in the nozzle, the nozzle becomes unusable until it is cleaned out, which is time consuming, difficult, and expensive.
Accordingly, it has been considered desirable to develop a new and improved injection molding machine sprue bushing and bushing and nozzle construction which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.